Weight classes differences aside? The weight difference is the main reason Toney would win. Duran was better at his best weight class (135, and also 147) than Toney at his best (168.) Toney was inconsistent at middleweight, getting a gift against Dave Tiberi and another close debatable decision over McCallum in the rematch (Toney won the first fight IMO as well as the Johnson fight), but Duran's MW resume isn't exactly the stuff of legend either. Winning 4-5 rounds of 15 against Hagler (regardless of how close the 3 blind judges had it), losing a close one to Hagler's half brother, and beating Iran Barkley isn't the best of achievements either. It was impressive given Duran's natural size and age at the time, but we're talking straight up. Toney by decision.
It's just too bad that Toney can't get himself back in fighting shape like Duran typically could. I don't care how out-of-shape a fighter gets between fights as long as he shows up to the fight in the best possible condition (I understand that this is not typically possible, but Duran seemed to pull it off many times - which makes him all that more amazing to me.)